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Tweaking The 502 Rat To 553 HP And 608 Lb-Ft Of Torque

Jeff Smith Oct 1, 2001

Step By Step

This is how a Deluxe GM Performance Parts ZZ 502ci/502hp engine starts life. The Deluxe version comes fully assembled with a dual-plane aluminum intake, an 850-cfm Holley carburetor, a water pump, an HEI distributor, plug wires, spark plugs, and a starter motor.

The ZZ 502/502 is based on the Gen VI version of the big-block Chevy. This short-deck, four-bolt main block features a monster 4.466-inch bore with reinforced deck surfaces to improve head-gasket sealing. Other features found in the Gen VI versions are a one-piece rear-main seal and accommodations for factory hydraulic roller tappets.

Swinging from the forged-steel rods is a set of forged-aluminum JE pistons with a slight dome and a 1/16-inch ring package. The rods are the same forgings used in the LS6/LS7 454 Rats with 7/16-inch rod bolts.

Combine 502 ci of Rat with oval-port heads and you’re guaranteed monster torque.

GM Performance Parts aluminum oval- port heads feature 2.25-/1.88-inch stainless valves and a 110cc combustion chamber. This chamber has already benefited from Todd McKenzie’s handiwork.

While these oval-port heads perform well right out of the box, we asked Todd McKenzie of McKenzie Cylinder Heads to tweak the area around the valve seats to pump the airflow. McKenzie then tested the heads on his SuperFlow 600 bench. The flow numbers illustrate how both the intake and exhaust flow improved, which is the main reason for the power increase on the dyno.

The deluxe 502/502 package comes with an interesting 850-cfm vacuum-secondary Holley carburetor equipped with an electric choke. This is a 4150-style carburetor with center-hung bowls and metering blocks on both the primary and secondary sides of the carb.

To ensure accurate internal clearances, Ed Taylor disassembled the 502 and took it to Jim Grubbs Motorsports (JGM), where the guys added new ARP rod bolts, resized the big ends of the rods, and torque plate–honed the cylinders. JGM also decked the block to ensure everything was square.

Once JGM completed the machine work, Taylor meticulously cleaned the entire engine and then carefully reassembled the short-block using new Federal-Mogul rings and bearings.

With the 502 reassembled, Taylor bolted the fat Rat to Ken Duttweiler’s dyno. We tested the Rat with both 1-3/4-inch and 2-1/8-inch Hedman headers and a Pertronix conversion of the stock HEI. We didn’t expect to see huge gains, but the combination surprised us with a stout 553 hp and a massive 608 lb-ft of torque with the big headers.

Even powerful Rats need to look good, so we bolted on a set of March Performance pulleys and alternator bracket to give our 502 a polished appearance.

We tried this 830-cfm HP-series carburetor and witnessed an increase in power—especially in the area below peak torque. This is a mechanical- secondary carb with a more sophisticated fuel curve that certainly proved its worth. At the horsepower peak, the differential was only 6 hp.

It’s a jungle out there. If you’re gonna be a player, you gotta carry a big stick. There are scads of ways to build horsepower, but the easiest is with a big-block. And if you’re going to stuff a Rat into your Camaro or Chevelle, you might as well go big. For us, that meant a GM Performance Parts 502ci/502hp. This engine fills the bill when size matters. The 500hp mark is the new power benchmark and the place where high performance is more than just an over-used term. While horsepower is important, what really got our attention was the 502’s driveshaft-twisting 567 lb-ft of torque.

But because our flag says Chevy High Performance, it’s tough to leave things the way they come in the box. We thought it would be fun to tweak one of these rotund rodents, so we convinced GM Performance Parts to ship us a Rat so we could tune it up…just a little.

We’ll save you the trouble of searching for the dyno-test figures and tell you that with open 2-1/8-inch Hedman headers on Ken Duttweiler’s dyno, the GM Performance Parts tuned-up Rat thumped out an honest 553 hp and 608 lb-ft of torque. Muffled with a pair of Borla stainless steel headers and smaller 1¾-inch Hedman headers, the Rat pumped out a more tame 527 hp but still grunted with 606 lb-ft of torque at an incredible 3,600 rpm! That’s tire-frying torque, folks.

But before we get into the specifics of how we tweaked this Rat, let’s take a look at the off-the-shelf GM Performance Parts Deluxe version of the ZZ 502/502.

Packin’ the Rat

Any racer will tell you that the ideal bore/stroke combination is a big bore and a short stroke. The large bore allows the heads to breathe, while the short stroke reduces piston speed and friction. Make that a 4.4660-inch bore and a 4-inch stroke and you have the ingredients for a 502. GM Performance Parts used a cast-iron block with siamesed bores to stretch the standard 4.251-inch 454 bore to a more rotund 4.4660 inches. A one-piece rear-main seal, stock 454-stroke crank pumps the displacement to 502 inches.

In a previous article, we outlined the differences between the older MK-IV block and the newest Gen VI versions, but one of the more critical differences is that Gen V and some Gen VI blocks come without a mechanical fuel pump boss, requiring an electric fuel pump to feed this monster. Of course, the one-piece rear-main–seal crank also demands its own specific flywheel/flexplate.

Heads and cam timing are the two major players that determine the power curve. Since the ZZ 502/502 is intended to be a hot street engine, the only limitations were respectable idle characteristics and the ability to run on 92-octane pump gas. The hydraulic roller cam offers decent duration that benefits from a healthy lift in excess of 0.500 inch. Squeezing a 9.6:1 compression ratio, this combination easily thumps at idle giving the 502 an authoritative tone.

While it may seem anticlimactic to stick a set of oval-port heads on a Rat this size, GM Performance Parts put considerable thought behind this decision. While rectangle-port heads were used on 396ci engines in the ’60s, that probably qualified as overkill for those smaller Rats. The squared-off oval-port GM Performance Parts aluminum heads do a great job of flowing air with help from 2.25-/1.88-inch intake and exhaust valves. In CHP’s big-block test back in the Apr. ’99 “Flow to Go, Part II” story, we discovered this large oval-port head dished up the best average flow of any Rat head with an intake-port volume of 200 to 299 cc’s. Even more impressive is an exhaust port that outflows many larger Rat-motor heads. All of this creates an impressive torque curve. The 502 thumps over 500 lb-ft of torque all the way from 2,750 to well past 5,000 rpm. This promises eyeball-flattening acceleration and just a little bit of fun.

Couple this with a dual-plane intake, an 850-cfm vacuum-secondary carburetor, and a rock-solid HEI ignition, and you have all the ingredients necessary for Rat-Fest 2001. While the box-stock 502/502 offers great power, we decided we would have to modify the motor if for no other reason than to leave our mark on our Grand Prize Rat.

The CHP Version

The ZZ 502/502 is plenty impressive right out of the box, but we thought it deserved some special attention since it would be given away as part of the promotional package that kicks off this month’s great new redesign. Since simplicity is our middle name, we left all of the major engine components in place, such as the cylinder heads, cam, and intake manifold, and instead focused on fine-tuning this thumper combination.

Power and reliability are an unbeatable combination, so our first move was to disassemble the engine and blueprint all the internal clearances. Jim Grubbs Motorsports fitted the rods with >> ARP bolts and resized the big ends, milled the deck, and torque plate–honed the block all in the name of idealized clearances. Grubbs also rebalanced the entire rotating assembly in search of optimal spin. Then Taylor reassembled the short-block with Federal-Mogul rod, main, and cam bearings as well as a 1/16-inch Federal-Mogul file-to-fit ring set. With the big bore, the 502s prefer a larger ring endgap, so Taylor increased the top to 0.024 and the second to 0.026 to ensure good ring seal.

Since this is a street engine, we asked Todd McKenzie from McKenzie Cylinder Heads to massage the heads just a little to tune up the flow. McKenzie responded with a quick pass of his grinder that resulted in respectable gains for both intake and exhaust ports, as you can see in the accompanying flowchart numbers. This is not a full-porting effort. Instead, McKenzie concentrated on the bowl area around the valve seats while also shaving the combustion-chamber wall slightly to unshroud the valves. The mid-lift flow numbers improved significantly.

In keeping with the spirit of the GM Performance Parts ZZ 502/502, we retained the stock hydraulic roller camshaft and stamped-steel 1.7:1 rocker arms all in the name of a streetable Rat motor. The only other change was a move to convert the stock HEI over to a Pertronix module, coil, cap, and rotor package. We also left room for a few minor cosmetic improvements. Taylor treated the block, pan, valve covers, and accessories to a coat of Plasticoat Orange paint and then dressed our Rat with a March Performance Products serpentine-belt accessory drive setup along with a ProForm chrome alternator and air cleaner.

The Test

Once Taylor had carefully reassembled the engine, it was ready for the ultimate test on Ken Duttweiler’s dyno. With the engine warmed up and carefully broken in and with a full load of 92-octane 76 pump gas, it was time to start making some heat. Taylor configured the 502 with a set of 1-¾-inch headers and a full 2-½-inch exhaust system complete with a set of Borla stainless steel mufflers. After a quick timing drill that found best total timing of 34 degrees, the 502 pumped out over 580 lb-ft of torque and 519 hp with some minor tweaking to the vacuum-secondary carburetor.

Since we had no prior experience with this carburetor, Taylor decided to try Holley’s new 830-cfm HP carburetor. While the stock 850-cfm Holley did not constitute a restriction, the HP-series carburetors offers a slightly better fuel curve that can be worth some power. After a quick jet check, Taylor pulled the handle on the dyno and cranked out an impressive 607 lb-ft of torque at 3,600 rpm, while the horsepower jumped to 527 at a loafing 5,400 rpm.

Taylor and Duttweiler concluded that the smaller headers and the McKenzie-modified oval-port heads were the contributing factors to the massive torque gain. They decided to try a set of 2-1/8-inch headers with open exhaust to see if they could squeeze a little more horsepower out of the fat Rat at the expense of a little torque. The result was an additional 26 hp at a slightly higher 5,500 rpm with a peak of 553 hp, while the torque remained strong at 608 lb-ft at 4,000 rpm.

These larger headers would be tough to squeeze into a stock Chevelle and would be especially tight in an early Camaro, so we would opt for the smaller set and just deal with the massive torque. Got a monstrous ’70 Impala or even a boat that thrives on torque? This is the engine to use. Better yet, plug this 502 into a tubbed Chevy with fat rear tires and let the motor try to bend the frame!


If you don’t believe that torque is king, then you’ve never ridden in a dialed-in big-block–powered car. It’s clear that the combination of a good-flowing set of oval-port heads on a 502ci short-block with a streetable cam is well worth the price of admission. If you’re not lucky enough to win this rascal, this story makes a great blueprint. Build it. You’ll be glad you did.


March Performance
Naples, FL 34110
Hedman Hedders
Whittier, CA 90602
Jim Grubbs Motorsports
Valencia, CA 91355
Holley Performance Products
Bowling Green, KY 42101
Southfield, MI 48034
(248) 354-7700
Ventura Motorsports
Ventura, CA 93002

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